Modern cognitive radio technology allows dynamic spectrum sensing, spectrum management, mobility and spectrum sharing, to mention a few. Classical cognitive radios change frequency channels when the interference level, or other parameters associated with operation, can be improved by moving to another frequency. Recent signal processing and multiple antenna technologies, however, allow expanded cognitive operation where receiver algorithms and transmit waveforms are dynamically adjusted for an operational environment.
Spectrum is a limited resource. A large amount of spectrum is required to deliver services that are associated with modern wireless personal communications. Typical examples are smart phones, wireless internet to laptops, and other such devices. These services consume a large amount of spectral resources causing both financial and spectrum policy issues.
Typically, these services are provided using licensed spectrum. The financial burden from licensing is billions of dollars, even for a relatively small amount of spectrum when compared to freely available unlicensed spectrum. The licensing, however, is required to make sure that current 1 to 4G radio technologies have the coordinated access they require to deliver a quality of service that is adequate for the end user application.
Currently in the United States there are several hundred MHz of unlicensed spectrum that can be used for delivering wireless services to consumers, however, traditional radio technologies typically suffer from interference from uncoordinated access from other unlicensed users. A novel radio technology is required that can deliver service while being highly resistant to interference and while also creating as little interference as possible to other users in the unlicensed band.
A state of the art cognitive radio network can use multiple frequency channels or bands automatically. This ability to create a network without detailed operator configuration allows for efficient use of spectrum, cost, and time, and also an efficient network deployment.
Self organizing and cognitive capabilities, however, require that the Access Points (AP) or Base Stations (BS) are able to measure the same and other network Access Points or Base Stations. In a TDD system a coordination of listening times is required among collocated transmitters. For example, APs that are closer than some predetermined distance need to be restricted from transmitting during measurement periods in a measured channel to avoid receiver front-end dynamic range limitations. The front-end overloading can happen if one device is transmitting on a frequency that is close to a frequency that the other device is using to receive weak signals.